As a form, it is known that an image forming apparatus comprises a decoloring apparatus which uses a recording material, for example, a decolorable toner to print an image on a sheet, and then executes a decoloring processing on the toner forming the image by heating to erase the image printed on the sheet. It is known that the decoloring apparatus, which includes a reading section configured to read an image to keep the image previous to being decolorized and a decoloring section configured to decolorize the toner forming the image, carries out an image reading processing with the reading section once again to determine whether or not the decoloring processing is normally executed after the decoloring processing. Thus, at the time the image formed on the sheet is decolorized, a series of operations, that is, reading and storing content of the sheet through the reading section and then decolorizing the content, are executed.
In recent years, a writing material the ink of which is colorless according to the change of temperature caused by heat is widespread. Such a writing material is used in a wide range from the use of check or memo to the use of drawing. In this way, due to the wide use, the area to be decolorized when the recorded content is useless is also increased, and the labor and the time are required in the decoloring job by the dedicated eraser. The cases of decolorizing an image with a duplicating machine that is arranged in the convenience store and capable of decolorizing an image through the heat are increased in order to decolorize the content recorded with the writing material. The decolorable ink of the writing material becomes colorless at about 65 degrees centigrade. In the duplicating machine, if the useless sheet is put into the sheet feeding stage to be subjected to a blank paper copy processing, the discharged sheet is decolorized because of being heated at a fixing temperature which is about 180 degrees centigrade for the general toner and is higher than the decoloring temperature. Thus, there is a problem that the memo and figures or the recorded characters in the critical document that are required to be left are also decolorized according to the relationship between the colorless temperature of the ink and the fixing temperature of the toner.